We report the real-space observation of magnetic domains in the ac-plane at the metamagnetic transition in a triple-layer ruthenate Sr₄Ru₃O₁₀ by magneto-optical imaging and scanning Hall probe measurements. We find anisotropic magnetic domain structures with weak c-axis correlation at low fields. The magnetic inhomogeneities are removed above the metamagnetic transition field. These changes of magnetic structure support the phase-separation scenario for the ultra-sharp steps in magnetoresistance.